The building of arches generally requires that a non-reusable wooden support structure or arch-forming assembly be prepared for constructing each arch. This approach is time-consuming and costly because the assembly materials are nonreusable. Austrian patent AU-B-69 705/81 teaches a reusable, arch-forming assembly which has a central hub positioned within the arch. A number of swivelable support arms resembling spokes are attached to this hub and extend away from it. Each support arm is swivelably or firmly attached to a flexible sheet at an anchoring point. The length of each support arm can be adjusted to set various arch shapes and/or spans. The arm adjustment process is time-consuming and cumbersome because a number of points along the arc have to be fixed such that the flexible sheet assumes the desired curvature.
A reusable arch-forming assembly having two parts is described in European Patent EP-A-01 36 906. The parts are hinged to make it possible to construct pointed arches. A plurality of support arms are swivelably mounted on a central hub and firmly attached to flexible sheets by their other ends. The hinge has a lever mechanism to enable the construction of round and circular arches. The adjustment of the multiple arms is cumbersome and time-consuming.
German Patent DE-A-22 60 847 discloses an adjustable arch-forming assembly with one flexible sheet carried by a number of support arms extending radially away from a central hub clamp. The length of the arms is adjustable. Varying the span and/or shape of the arch in this arch-forming assembly is time-consuming and difficult.
In an adjustable arch-forming assembly for making section arches or pointed arches described in German Patent DE-A-23 34 020 a number of spoke-like support arms are fastened to a central hub clamp. Each support arm has joint near the flexible sheet, such that the end of each arm can be pivoted with respect to the flexible sheet at the joint. The sheet has slidable terminal portions for making section arches or arch segments. These terminal portions are also supported by arms. Varying the shape and/or span of the arch is time-consuming and cumbersome.